Our invention relates to a method of making a pressible mixture of lignocellulose-containing fibers and a thermoplastic material.
One such method is already known and described in German Patent Document DE-A-32 30 888.
For some time and to an increasing extent molded parts, for example semifinished panels made from thermoplastic material and glass fiber, have been manufactured for use in automobile production.
Specifically a glass fiber-thermoplastic molded part, also GMT(Glass Mat Reinforced thermoplastic), has been made in the following way: First, a semifinished panel made from glass fibers and a thermoplastic material is produced in a laminating press. This semifinished panel is heated in a second process step outside of the press until at the softening temperature of the thermoplastic and subsequently is pressed in a cooled press under high pressure to form a molded part. In this method usually the blank for the semifinished panel is smaller than the size of the already finished pressed molded part, i.e. the material of the semifinished panel flows simultaneously in the pressing operation. Thus the socalled flowing pressing occurs.
Molded parts for the already mentioned and additional applications may be made with the same quality economically, if in making the semifinished panels lignocellulose-containing fiber are used instead of glass fibers. The lignocellulose-containing fibers may be comminuted, cut up and dried wood chips, bagasse fibers and the like. It is however difficult to make semifinished panels from the lignocellulose-containing fibers and a thermoplastic material, since the lignocellulose-containing fibers contain a natural resin portion, which prevents the fibers from attaining a tight, strong bond with the thermoplastic material.
In the previously mentioned process to overcome this difficulty, the lignocellulose-containing fibers are exposed to ionizing radiation, e.g. gamma radiation, prior to mixing with the thermoplastic. It is questionable whether such a treatment is feasible on a large scale and, even if it were, such a method of manufacture of thermoplastic-wood fiber semifinished panels would be costly.